This invention relates to a method and system to measure the load attached to a crane hoist. More particularly, the invention relates to a process for measuring the load lifted by a crane hoist by utilizing a parameter adaptation that uses drive speed and torque feedback as inputs.
Many types of cranes are used to move loads in a wide variety of environments. In particular, container cranes are used to lift and move a wide range of loads between a ship and a dock. Illustratively, a container crane may include a crane structure, a drive system, a wire rope or cable, and a lifting device to connect to a container, for example. The crane structure may take on a variety of forms such as a trolley, a boom structure or a girder structure. The crane structure is movable such that a load may be raised, moved as necessary, then lowered to the desired position.
The conventional container crane includes a drive system that controls the hoisting of the load using a wire rope or cable. The drive system may include a hoist motor and a gearbox connecting the hoist motor to a hoist drum. The wire rope or cable is coiled on the hoist drum such that the wire rope may be payed out from or coiled upon the hoist drum as is known in the art. In a container crane system, the wire rope runs from the hoist drum through the crane structure to the lifting device. The wire rope may be of any suitable construction and is typically a steel cable.
The container crane system includes a lifting device. Illustratively, the lifting device may be a spreader or a cargo beam, for example. The spreader is commonly used in hoisting containers, for example. The spreader commonly includes twist locks to attach the spreader to the container. A different type of lifting device such as the cargo beam may be used for heavier loads. The cargo beam may be used in conjunction with slings. Illustratively, a boat may be hoisted and moved using a cargo beam lifting device.
The primary objective of a crane is to move a load from a first position to a second position. It is common for a crane design to make use of constant power operation of the hoist. This allows lighter loads to be moved at higher speeds, while heavier loads are moved more slowly. As a result, this increases the efficiency of the hoist without creating a need for a very large hoist motor design. Accordingly, the load must be measured dynamically in order to compute the maximum safe speed at which the load may be moved, according to the constant power curve. In performing this objective and in order to optimize efficiency, the container crane must measure the weight of the load as quickly as possible, while providing limited overshoot in the measurement.
However, one difficulty arises in measuring the load accurately while accelerating the load. In known container cranes, two methods are presently used to measure a lifted load. A first method includes the utilization of load cell feedback. This method accepts a load indication from the load cells placed on crane sheaves or headblock of a crane hoist, for example. However, due to accelerating forces, the signal generated from the load cell is typically inaccurate during changes in speed of the hoist. As a result, the load is more slowly accelerated to the maximum safe speed for that load.
A further known method utilizes drive speed and torque feedback to measure the load. In this additional method, the speed feedback is filtered, differentiated, and multiplied by inertia to approximate acceleration torque. A loss profile is programmed to approximate the frictional losses in the system as a function of the speed or some other variable. These frictional losses are subtracted from the torque feedback signal to provide an indication of load torque, which is then filtered and scaled to provide the lifted load. This additional method provides good accuracy in a steady state, but experiences errors during changes in speed because of the difficulty in differentiating speed feedback to approximate acceleration. Accordingly, this second method cannot be adjusted to respond fast enough and with enough accuracy to provide any protective features which might be utilized in operation of the crane hoist.
Another, slightly different way to do this is to filter and differentiate a speed reference instead of speed feedback. This has similar dynamic problems because it cannot remain accurate if the hoist drive hits an electrical current or torque limit. However, it is common for a crane hoist to hit an electrical current or torque limit because the crane hoist system is sized to use all available current to provide the best performance possible.
Accordingly, as described above, drive current and speed feedbacks can be used to determine the load lifted by a container crane, for example. Once the load is known, the maximum safe speed of operation is determined. However, the conventional techniques can be unstable and always rely heavily on the tune-up of the hoist motor control system that is utilized.
Thus, there is a particular need for a crane hoist system to overcome these problems. Briefly, in accordance with one embodiment of the present invention, a parameter adaptation method utilizes a model of the physics of the crane hoist system in order to measure the weight of a lifted load. The system and method of the invention utilize the model of a crane hoist system and the on-line parameter adaptation technique to measure the load accurately and with a faster response than was possible in known systems.
In the invention, the load measurement process measures the lifted load by a crane hoist by applying an on-line parameter adaptation. The parameter adaptation utilizes drive speed feedback of the hoist motor system and torque feedback of the hoist motor system as inputs. In particular, the parameter adaptation filters the drive speed feedback and the torque feedback of the hoist motor system, and processes these filtered values in conjunction with a previously determined load measurement. Additionally, the method and system of the invention provide the accuracy, as well as the speed of response, to detect certain fault conditions of the hoist. Specifically, the process of the invention allows for slack cable detection, overload protection, as well as snagged load detection.